Pulping. The transition of a tree into paper involves several discrete stages. Stage one is the debarking of the tree and the conversion of the tree into wood chips. Stage two is the conversion of wood chips into pulp. This conversion may be by either mechanical or chemical means.
Bleaching. Bleaching is the third stage. Delignification is the first step in the bleaching of chemical pulps. Lignin, a complex polymer derived from aromatic alcohols, is one of the main constituents of wood. During the early stages of bleaching, residual lignin, which constitutes 3-6% of the pulp, is removed. Currently, this is typically done by treatment of the pulp with elemental chlorine at low pH, followed by extraction with hot alkali. Once a significant portion of the residual lignin has been removed, the pulp may be whitened, by a variety of means, to high brightness. Chlorine dioxide is commonly used in the brightening step.
Although chlorine compounds are effective and relatively inexpensive, their use in pulp mills results in the generation and release of chlorinated organic materials, including dioxins, into rivers and streams. Due to increasing regulatory pressures and consumer demand, new, non-chlorine bleaching technologies are urgently needed by manufacturers of paper-grade chemical pulps.
Chlorine-Free Bleaching using Polyoxometalates.
Polyoxometalates are discrete polymeric structures that form spontaneously when simple oxides of vanadium, niobium, tantalum, molybdenum or tungsten are combined under the appropriate conditions in water (Pope, M. T. Heteropoly and Isopoly Oxometalates Springer-Verlag, Berlin, 1983). In a great majority of polyoxometalates, the transition metals are in the d.sup.0 electronic configuration which dictates both high resistance to oxidative degradation and an ability to oxidize other materials such as lignin. The principal transition metal ions that form polyoxometalates are tungsten(VI), molybdenum(VI), vanadium(V), niobium(V) and tantalum(V).
Isopolyoxometalates, the simplest of the polyoxometalates, are binary oxides of the formula [M.sub.m O.sub.y ].sup.p-, where m may vary from two to over 30. For example, if m=2 and M=Mo, then the formula is [Mo.sub.2 O.sub.7 ].sup.2- ; if m=6, then [Mo.sub.6 O.sub.19 ].sup.2- ; and if m=36, then [Mo.sub.36 O.sub.112 ].sup.8-. Polyoxometalates, in either acid or salt forms, are water soluble and highly resistant to oxidative degradation.
Heteropolyoxometalates have the general formula [X.sub.x M.sub.m O.sub.y ].sup.p- and possess a heteroatom, X, at their center. For example, in the .alpha.-Keggin structure, .alpha.-[PW.sub.12 O.sub.40 ].sup.3-, X is a phosphorus atom. The central phosphorus atom is surrounded by twelve WO.sub.6 octahedra.
Removal of a (M=O).sup.4+ moiety from the surface of the .alpha.-Keggin structure .alpha.-[PM.sub.12 O.sub.40 ].sup.3- where M is molybdenum or tungsten, creates the "lacunary" .alpha.-Keggin anion, .alpha.-[PM.sub.11 O.sub.39 ].sup.7-. The lacunary .alpha.-Keggin ion acts as a pentadentate ligand for redox active transition-metal ions, such as vanadium(V) in .alpha.-[PVW.sub.11 O.sub.40 ].sup.4-. Further substitution is also possible, giving anions of the form [X.sub.x M'.sub.m M.sub.n O.sub.y ].sup.p-, such as .alpha.-[PV.sub.2 Mo.sub.10 O.sub.40 ].sup.5-. In place of vanadium(V), d-electron-containing redox active transition-metal ions (TM), may also be used, giving complexes such as .alpha.-[SiMn(III)(H.sub.2 O)W.sub.11 O.sub.39 ].sup.5-,which contains a manganese(III) ion. While stabilizing the active metal ions in solution and controlling their reactivity, the heteropolyanions are highly resistant to oxidative degradation (Hill, et al., J. Am. Chem. Soc. 108:536-538, 1986).
Effluent Free Mill. During delignification or bleaching, whether by chlorine, chlorine dioxide, oxygen, hydrogen peroxide, ozone, or other methods, lignin and polysaccharide fragments are liberated as water-soluble organic compounds. After delignification or bleaching, these compounds remain dissolved in the liquor. At present, water soluble lignin and polysaccharide fragments removed from wood pulps during bleaching are generally treated in biological waste-treatment ponds prior to their release to rivers and streams. Unfortunately, biological remediation fails to remove or to sufficiently degrade all of the dissolved organic materials present. As a result, potentially harmful organic compounds, particularly those generated during chlorine bleaching, are released into the environment. Because some of the materials that survive the biological waste treatment may have deleterious environmental effects, there is a need for alternative and more effective methods for degrading these materials.
Many in the U.S. pulp and paper industry expect that the release of any organic waste (other than carbon dioxide) into the environment will eventually be banned altogether. There is thus an additional need for the development of a "closed" bleach mill from which few or no chemical waste-products, other than carbon dioxide and water, are released (Pulp and Paper Mill of the Future--An Information Exchange, U.S. Department of Energy, Office of Industrial Technologies, Orono, Me., September 8-10, 1993). As disclosed below, polyoxometalates are employed as reusable oxidizing agents or catalysts for selective bleaching of wood pulps. As reusable agents, the polyoxometalates are suitable for repeated use in a closed mill. During polyoxometalate bleaching, however, residual kraft lignin fragments, and some polysaccharide fragments, are dissolved by the polyoxometalate bleaching liquor.
What is needed in the art of polyoxometalate bleaching is a method for achieving mill closure by removing dissolved lignin and polysaccharide fragments from the bleaching liquor.